IST = inapropriate sinus tachycardia
Back in the 19th century, there used to be a condition called “neurasthenia”. Young women (the beautiful but delicate ones, according to the romance novels of the time) would find themselves suddenly unable to function due to a host of inexplicable symptoms, often including fatigue, weakness, strange pains, dizziness, and passing out. Doctors would not find anything to explain these symptoms, so they were attributed to a “weak nervous system”, or neurasthenia. Victims were often confined to their beds, where they would either recover or, eventually (and tragically) die. And while nobody knew what caused this condition, everyone —doctors and laymen alike—took it seriously; the condition and its sufferers were treated with great respect.
Today’s doctors shake their heads in wonder at the notion of such a condition, and tend to put neurasthenia in the same bucketas the witchcraft hysteria of a few centuries earlier. But patients who would have been called neurasthenics 150 years ago are still around; they’re just given different labels. These labels include chronic fatigue syndrome (CFS), vasovagal or neurocardiogenic syncope, panic attacks, anxiety, irritable bowel syndrome (IBS), postural orthostatic tachycardia —and, quite possibly, IST. While most syndrome (POTS), fibromyalgia doctors still tend to think of these various syndromes as stand-alone conditions, they are all part of a general class of conditions called the dysautonomias.
In dysautonomia the autonomic nervous system loses its normal balance, and at various times the parasympathetic or sympathetic systems inappropriately predominate. Symptoms can include frequent, vague but disturbing aches and pains, faintness or frank syncope, fatigue and inertia, severe anxiety attacks, sinus tachycardia, hypotension, poor exercise tolerance, gastrointestinal disturbances, sweating, dizziness, blurred vision, numbnessand tingling, anxiety and (quite understandably) depression.
Sufferers of dysautonomia can experience all these symptoms or just a few of them. They can experience one cluster of symptoms at one time—and another at other times. And since people with dysautonomia are usually normal in every other way, a physical exam most often does not reveal any abnormalities. Patients are often labeled hysterical and are accorded little of the respect they received during the 19th century. (Fortunately, doctors no longer prescribe bed rest, so the risk of mortality is now very low.) When patients do get an actual diagnosis, the one they receive does often depend on their recently dominant symptoms and on which specialist they are referred to.
What causes dysautonomia? The dysautonomias do not have a single cause. Some patients inherit the propensity to develop dysautonomia syndromes, and variationsof dysautonomia often run in families. Viral illnesses can trigger a dysautonomia syndrome. So can exposure to chemicals. (Gulf War Syndrome is, in effect, dysautonomia—low blood pressure, tachycardia, fatigue and other symptoms—that, government denials aside, appears to have been triggered by exposure to toxins.) Dysautonomia can result after various types of trauma, especially trauma to the head and chest. (It has been reported to occur for example after breast implant surgery.) Dysautonomias caused by viral infections, toxic exposures, or trauma often have a rather sudden onset. CFS, for instance, most classically begins following a typical viral-like illness (sore throat, fever, muscle aches, and so on), but any of the dysautonomia syndromes can have a similar onset.
Is IST one of the dysautonomias? Obviously we do not know for sure, but it certainly shares many of the characteristics of dysautonomia, including that its onset is frequentlypreceded by a viral illness or trauma; that the patient profile is typical; and that “extra” symptoms frequently occur which are consistent with other forms of dysautonomia. (Indeed, many IST patients might have beenlabeled as suffering from IBS, POTS or CFS if they had seen someone other than an electrophysiologist.) Further, the fact that something stimulates the successfully ablated SA nodes to regenerate in IST patients suggests a more systemic problem than intrinsic SA nodal disease. And finally, electrophysiologists have noted that symptoms consistent with dysautonomia often persist even after successful SA nodal ablation (i.e., during the period of time that normal heart rates have been restored).
From: Richard N. Fogoros, Electrophysiologic testing, 4th ed.,2006.